In our area, the ductwork, HVAC equipment and water heating equipment are most often in the attic. We have virtually all flex spider systems. Wall chases are frequently used for returns and supplies. Point a thermal camera on a wall and you are likely to see at least half the wall and ceiling light up. The top plates are rarely sealed. In fact, we almost never see anything sealed, hence the numerous complaints of black streaks on walls and 'mold' on grills. Hopefully, you have more hospitable conditions.